Code Division Multiple Access (CDMA) is a multiple access method based on spread spectrum used in cellular communication systems. In CDMA, the narrow band data signal of a user is spread across a relatively wide frequency band using a spreading code having a broader bandwidth than the data signal. Typically, many users transmit simultaneously using that same frequency band. An individual spreading code is also used on each connection between the base station and the mobile station so that individual user signals may be distinguished from each other at a receiver based on the user's spreading code. Mutually orthogonal spreading codes are desirable because they do not correlate with each other. In practice, the spreading codes are not completely non-correlated, and the signals of other users complicate the detection of the desired signal by distorting the received signal. The mutual access interference caused by simultaneous users is a key factor affecting the capacity of a CDMA cellular communication system. The interference may be reduced by attempting to keep the transmission power levels of mobile stations as low as possible using Transmit Power Control (TPC).
Many types of communication systems therefore rely on power control to maintain a desired quality of service. TPC is typically achieved by having the receiver measuring the received Signal-to-Interference Ratio (SIR), comparing the measured SIR with a desired SIR and finally adjusting the transmit power up or down to minimize the difference between the desired and the measured SIR.
In WCDMA (Wideband CDMA), an inner power control loop performs this adjusting, while an outer loop power control is used to adjust the desired SIR to reflect the actually desired quality of service. Such quality of service is most often defined as a desired block error rate or the desired number of retransmissions. The outer loop is typically relatively slow, having a typical update rate of 10 to 100 Hz in WCDMA.
The transmit power is typically adjusted by the inner loop on regular basis. In WCDMA the update rate is 1500 Hz. Typically, a TPC command is used to inform the transmitter to change its power. In order to save signaling resources, these TPC commands are typically binary and thus it can only ask the transmitter to increase or decrease its output power with a predetermined amount (typically 1 dB). Furthermore, there will be a delay associated with the TPC loop as it will take some time to measure the SIR, to transmit the TPC command, to demodulate the TPC command, and to update the transmit power. In WCDMA this delay is typically 2-3 ms.
Without TPC the SIR would vary due to varying interference and the fading rate of the radio channel transmitter. The ability of the transmit power control to maintain the desired SIR is limited by the update rate, the delay in the loop and the size of the power control step. Power control is essential for e.g. WCDMA Uplink (UL) to maintain coverage and capacity. Furthermore, with EUL and increased data rates, interference cancellation is envisioned to play an important role to enable these high data rates. The area of advanced receivers and specifically interference suppression or cancellation will be very important for reaching working conditions to secure high system capacity when allowing multiple high data rate users to be active. The associated area of issues and aspects of control handling as the delay or quality of system commands is important to cover and secure IPR within to secure efficient usage and achieving the performance potential of the more advanced receiver techniques from a system point of view.
Interference Cancellation (IC) can be used to improve the SIR. With IC, part of the interference is typically demodulated and regenerated and then cancelled before demodulating other user's signals. Ideally, it would be beneficial to use such improved SIR for reducing the transmit power and thereby allowing for additional traffic and control signaling resources to be used. However, using the SIR values obtained by conventional IC methods implies quite a significant delay. The delay is due to that it takes time to process the IC. In postcoding IC, the received signal of a Time Transmission Interval (TTI) is first decoded. Interference signals are thereafter regenerated and cancelled. This implies that when post-coding IC is employed, a complete Time Transmission Interval (TTI), e.g. 2 or 10 ms for Enhanced Uplink (EUL)/WCDMA, of the signal to be cancelled, must be received before that signal can be decoded, regenerated, and cancelled. Thus, the minimum delay for post-decoding IC corresponds to one TTI of the signal to be cancelled. It is important for the TPC to use an SIR estimate that reflects the SIR as experienced when demodulating the data channel, which can carry data as well as voice traffic. This would significantly limit the ability of TPC to follow the varying conditions of the channel, e.g. fading and interference, and, will for some cases cause oscillations in mobile station transmit power.
The most straightforward solution to handle this delay due to IC is to ignore the delay and have the TPC to operate as usual without any knowledge about the IC. However, then the measured SIR will not reflect the gains from applying IC. The problem is then that the overall gain with IC will be very small as the TPC will not be able to take into account the SIR improvements achieved by the applying IC. The outer loop power control could slowly adjust the SIR target in order to compensate for this. However, adjusting the out loop is a very slow process and constraints on control channel performance may limit the actual performance. Control channel performance could be addressed by adjusting static control channel power settings.
However, it is not desirable to have power settings that are depending on the algorithm used.